1. Field of the Invention
The invention relates to electrophotographic printing processes and in particular relates to liquid developer and associated apparatus and processes for creating a latent image on a photosensitive element of a printing system.
2. Discussion of Related Art
Electrophotographic printing systems are used to generate high quality printed images on ordinary paper or other media. Such electrophotographic printing systems are used in copying devices to duplicate images, facsimile machines to receive images and print them on paper and printing devices attachable to computer systems to present printed output from a computer program. Such electrophotographic printing systems are also referred to herein broadly as electrophotographic imaging systems, EP imaging systems, or simply imaging systems or EP systems.
In general, such EP systems include a photosensitive conductor surface--typically in the form of a rotating drum or belt. The photosensitive conductor (also referred to herein as photoconductor or PC) is a dielectric material that holds a charge. The charge retention of the PC is proportional to its exposure to a light source. A precision light source is therefore used to controllably expose individual points on the PC surface. Each such point is referred to as a picture element or pixel. Each pixel therefore represents a light or dark spot on an image to be printed on paper. The light or dark character of the pixel is proportional to the charge retained at that point.
The surface of the PC is said to contain a latent image. The collection of charged and uncharged pixels represents the image to be transferred to the paper or other printing medium. A developer component is associated with the PC component to apply oppositely charged toner particles to the PC. The latent charged image is said to be developed by transfer of the toner particles to the PC. The latent image is then represented by the charged toner particles electrostatically held to the PC.
The developed latent image is then transferred to a piece of paper or other medium by another electric field generated by a transfer component associated with the PC and developer component. The transfer component generates an oppositely charged field on the paper to cause the toner particles to transfer to and electrostatically adhere to the sheet of paper.
A fixing component then fixes the toner particles to the paper by appropriate heating and/or pressure.
The combination of the PC, the developer component and the transfer component is referred to herein as an imaging station. The fixing component may also be grouped into the term imaging station where there a one to one correspondence of fusing components to PCs. As noted below, however, there are situations where there may be multiple PCs (multiple imaging stations) that share a common fixing component.
Some EP systems include multiple PC components (and associated developer and transfer components) to enable multiple color printing. Each of the multiple imaging stations is responsible for a different toner imaging process. For example, a full color printer typically utilizes four PC components--one for each of the three primary colors and one for black. A highlight color printer may use two PC components--one for black and one for a highlight color or MICR (magnetic toner). A number of factors in the process may dictate whether such multiple imaging stations each include a fixing component or whether the imaging stations share a fixing component.
Many electrostatic developing systems use dry particle toners to create toned images on the PC. However, dry particle toners have numerous disadvantages. Because small dry toner particles become readily airborne, causing health hazards and machine maintainability problems, their diameters are seldom less than 3 microns, which limits the resolution obtainable with dry toner particles. Further, thick layers of dry toner, such as are necessary in color images, cause significant paper curl and thereby limit duplex applications. Therefore, there has been a great desire to develop liquid development components for EP printing systems.
Liquid ink development systems are generally capable of very high image resolution because the toner particles can safely be ten or more times smaller than dry toner particles. Liquid ink development systems show impressive gray scale image density response to variations in image charge and achieve high levels of image density using small amounts of liquid developer. Additionally, the systems are usually inexpensive to manufacture and are very reliable. However, present liquid ink development systems are based on volatile liquid carriers and, as a result, they pollute the environment. Consumers are often wary about using such liquid development systems for fear of health hazards. Therefore, there is a strong desire for a liquid ink development system that does not create airborne pollution.
Current liquid ink developers generally utilize low viscosity and low solids. Lower viscosity inks with lower volumes of solids (pigment and associated binders) give rise to two additional problems. First, maintaining uniform dispersion of the pigment particles is more difficult in a low viscosity carrier. The pigment particles have a tendency to drift and settle in the carrier liquid. Furthermore, low volume of solids in the ink increases the amount of ink required to generate a given latent image. More ink will have to be transferred to the photoconductor in order to provide sufficient pigment particles for a desired image density.
Larger amounts of carrier liquid associated with required larger volumes of ink creates the additional problems of transferring the higher volume of material to the photoconductor surface and removing the excess liquid carrier from the printed image. The higher volume of ink required can be a limiting in the print speed of the printing system.
In addition, present liquid ink developers often utilize highly volatile liquid carrier systems such as hydrocarbons. This results in the need to recover or vent the carrier fluids to avoid environmental problems in a common office work environment. In addition, such chemically active carrier liquids can cause harm to components of the image generation system. For example, an organic photoconductor material may be damaged over time from use of such chemically active carrier liquids.